Erasing previously entered characters in insert mode

In insert mode, to allow the backspace key to erase previously entered characters, autoindent, and newlines, set the following option:

:set backspace=indent,eol,start

Or:

:set backspace=2

By default this option is empty, not allowing you to backspace over characters that weren't entered during the current insert mode session, the automatically inserted indentation, or across line breaks. This is the standard vi behavior.